When using "ios.alert_accept()" it's showing "wda.exceptions.WDAUnknownError: WDARequestError(status=110, value={'error': 'unknown error', 'message': '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'})" error for latest ios-target

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**Describe the bug**
When using "ios.alert_accept()" it's showing "wda.exceptions.WDAUnknownError: WDARequestError(status=110, value={'error': 'unknown error', 'message': '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'})" error for latest ios-target

A clear and concise description of what the bug is. Or paste traceback below.

**To Reproduce**
Steps to reproduce the behavior:
1. create an object for class ios in script like:
ios = IOS("http://127.0.0.1:8100")
2. call the function "ios.alert_accept()" to the script
3. Run the script

**Expected behavior**
Must accept the alert pop-up shown on the iOS device

**python version:** `python3`

**airtest version:** `1.2.14`

**Smartphone (please complete the following information):**
- Device: iPad 6th Gen
- OS: iOS v 14.7.1
- ios-Target latest release version on 28th April
- Xcode v13.4
- MacOS 12.4

**Additional context**
The same script with "ios.alert_accept()" works when I use the "Appium WebDriverAgent"